"Restless" comes on the heels of more than a few films about hidden wartime pasts, most of them about Nazis. Not so in this richly textured World War II spy-thriller based on a novel by William Boyd. Its heroine Sally Gilmartin (Charlotte Rampling) is, like all her spy colleagues, devoted to the Allied cause, as the film makes amply clear from the start. It doesn't take much longer to recognize that the story she has to tell is a deeply sinister one. To know that, it's only necessary to look at Sally in the opening scenes, set in the mid-1970s, as she reveals her past to her astounded adult daughter, Ruth (Michelle Dockery). It's hard to think of another actress with as formidable a gift as Ms. Rampling's for exuding a sense of menace. A power all the more potent in characters like Sally—that is, proud, assertive women, self-possessed and, in one way or another, haunted.

It's a testament to the crackling intelligence of the script (written by Mr. Boyd) that the nature of that menace hangs elusively in the air until the end. Still, the real heart of this two-part series lies in the dazzling action scenes involving a British secret-service unit, told in flashback and set in splendidly evocative wartime (and prewartime) backgrounds—Paris, London, Washington, and there's a brief stop at 33rd Street and Third in New York. That's not to mention a long solo drive down a frightfully isolated road in New Mexico, past a prominent roadside sign announcing the way to Alamogordo—a name with a certain ring to it, Alamogordo having been the site of the first atomic-bomb test.

The driver of that car had been Sally's younger self, British agent Eva Delectorskaya, Sally's real name. As her daughter, and we, learn at the outset, the person known as Sally Gilmartin had been born of Russian parents, had been a member of a British secret-service unit during the war and an espionage agent. One taught to use her wits, go behind enemy lines, and find her way back alone in strange terrain, miles from safety—the section on the spy training school and its training exercises is itself worth tuning in for, if only for the blood chilling scenery—and to dispense with the enemy physically when necessary. Agent Eva's wartime history, revealed in those flashbacks (Hayley Atwell plays the young Eva), includes some spectacular instances in which she's shown doing just that.

Thirty years after the war, she's preparing to use her wits again, to go behind enemy lines once more, so to speak, and even to take physical action against a foe. She's in danger from certain enemies over matters related to her wartime work, people determined to kill her, whose identities she will not disclose till the very end. Fearful—and much to her daughter's dismay—this widow of a respected academic visits the local arms dealer to buy protection. She knows guns well, as she shows during that visit—an encounter eloquent in its detail, in her expertise on weaponry and bullets on casual display in her discussion with the gun dealer. A scene like numerous others in the show whose small moments deliver a large quotient of the heft and color in "Restless."

When it comes to color, and for that matter heft, none of the characters, Ms. Rampling's Sally aside, is the equal of Lucas Romer. He's played by Rufus Sewell, who brings majestic verve to the role of the dashing spymaster and unrelenting disciplinarian, and also a boss who becomes something more than a friend to young Eva—one of the few revelations allowable about this drama perilously awash in potential spoilers. There's not much, to be sure, likely to spoil the pleasures of this work, one of whose charms is a refreshingly unhurried air even as it goes ripping sharply along, suspenseful to the end.